In recent years, developments of electric vehicles (EV), hybrid electric vehicles (HEV) and fuel cell vehicles (FCV) have been advanced against the background of escalating environmental protection movement. For a power source for driving the motors used on those vehicles, the rechargeable secondary battery is suitable. In particular, what is attracting the attention of the market is a non-aqueous electrolyte secondary battery such as a lithium-ion secondary battery expected to provide high capacitance and high output.
A non-aqueous electrolyte secondary battery is provided to have a positive electrode activating material layer that is formed on the surface of a charge collector and includes a positive electrode activating material (it is possible to cite LiCoO2, LiMn2O4, LiNiO2 and the like, for example). Additionally, the non-aqueous electrolyte secondary battery is provided to have a negative electrode activating material layer that is formed on the surface of another charge collector and includes a negative electrode activating material (it is possible to cite metal lithium, carbonaceous materials such as coke, natural or synthetic graphite and the like, metal materials including Sn, Si and the like and oxides of these, for example). Furthermore, the non-aqueous electrolyte secondary battery is provided to have an electrolyte layer containing an electrolyte, the electrolyte layer being disposed between the positive electrode activating material layer and the negative electrode activating material layer to separate the positive electrode activating material layer and the negative electrode activating material layer.
In such a non-aqueous electrolyte secondary battery, a chemical reaction or decomposition of these electrolyte layers may occur at the surface of the positive and/or negative electrode. This has hitherto resulted in a problem such as reduction of the storage characteristics of battery at high temperatures and reduction of the cycle property of secondary battery, and additionally a problem of the generation of gas due to the decomposition product, and so on.
In order to prevent the occurrence of the problems, there has been employed the addition of a compound having a protective film-forming function to an electrolyte solution contained in the electrolyte layer. More specifically, it is known that, when the decomposition of the compound added to the electrolyte solution is deliberately accelerated at the surface of the negative electrode activating material at the time of the initial charging, the decomposition product forms a protective film having a protective function of preventing a further decomposition of the electrolyte layer, i.e., SEI (Solid Electrolyte Interface). Since the protective film is thus formed, a chemical reaction or decomposition of the electrolyte layer is suitably prevented at the surface of the negative electrode and results in achieving the effect of maintaining the battery performance of secondary battery, which has variously been reported.
For example, it is disclosed that a dicarbonyl compound or alternatively a dicarbonyl compound and a vinylene carbonate or the like are contained in a nonaqueous electrolyte solution thereby improving the cycle property and the storage characteristics of battery (see Patent Publication 1).